The properties of several enzymes associated with the endoplasmic reticular membranes of rat liver and kidney have been studied as a function of the age of the animals, with special emphasis on changes in latency. The latencies of two catalytic activities associated with the same enzyme, glucose-6-phosphatase, were found to show strikingly different age related changes in liver but not in kidney. The latency of PPi-glucose phosphotransferase activity reaches high levels (60-80% latent) soon after birth and remains high throughout life, while the latency of glucose-6-P phosphohydrolase decreases with age. The phosphohydrolase is 2 to 3 times more latent in the liver of the neonatal animal than in the adult.